Separation of c9 aromatic hydrocarbons



Nov. 30, 1965 E. M. AMlR ETAL 3,220,933

SEPARATION OF C AROMATIC HYDROCARBONS Filed April 6, 1964 LlGHTCOMPONENT MAKE-UP SOLVENT EXTRACTIVE -/-DIST|LLAT|ON ZONE HEAVYCOMPONENT M-AND P-ETHYLTOLUENE O-ET HY LTOLU ENE MESITYLENE PSEU DOCUMEN E FIG. 2. AND HEAVIER MA KE-UP INVENTORS- SOLVENT 4B EMANUEL M- AMIR,

WILLIAM R.EDWARDS ATTORNEY.

United States Patent 3,220,933 SEPARATION 013 C AROMATIC HYDROCARBONSEmanuel M. Amir, Baytown, Tex., and William R. Edwards, Mountainside,N.J., assignors, by mesne assignments, to Esso Research and EngineeringCompany, Elizabeth, N ..l., a corporation of Delaware Filed Apr. 6,1964, Ser. No. 357,599 6 Claims. (Cl. 20239.5)

This application is a continuation-in-part of Serial No. 38,067, filedJune 22, 1960, entitled, Separation of C Aromatic Hydrocarbons, forEmanuel M. Amir and William R. Edwards, now abandoned.

The present invention is directed to a method for separating aromatichydrocarbons. More particularly, the invention is concerned with theextractive distillation of C aromatic hydrocarbons. In its more specificaspects, the invention is concerned with separating C aromatichydrocarbons under vapor-liquid contacting conditions with the polyalkylesters of an aromatic polycarboxylic acid having adjacent carboxylicgroups.

Some 0,, isomers of aromatic hydrocarbons boil so closely together thatit has been extremely diflicult, if not impossible, from a practicalviewpoint to separate these particular aromatic hydrocarbons. Anexpensive superfractionation of the isomers can result in separating theisomers into a mand p-ethyltoluene fraction and a mesitylene,o-ethyltoluene fraction. However, the desired C isomers are mesityleneand o-cthyltoluene as separate fractions. In accordance with the presentinvention, these C isomers of aromatic hydrocarbons are separated byextractive distillation techniques even though their respective boilingpoints are very close together as illustrated in Table I below.

Table 1 BR, F. m-Ethyltoluene 322.4 p-Ethyltoluene 323.6 Mesitylene328.5 o-Ethyltoluene 329.3

It is noted from Table I that the boiling points of these isomers arewithin a few degrees Fahrenheit of each other. The boiling pointdifference between mesitylene and oethytoluene is less than one degreeFahrenheit. Using the extractive distillation techniques, which will bemore fully described hereinafter, an enrichment of the mesitylenefraction and the o-ethytoluene fraction can be made. Furthermore,according to another aspect of the present invention, separation of themand p-ethyltoluene can be made from the mesitylene by an easy andpractical extractive distillation. Throughout the description of thepresent invention, the designation mand p-ethyltoluene is to beconsidered as a mixture of the isomers recovered as a single, separatefraction.

In accordance with one aspect of the present invention, an extractivedistillation solvent has been found which will separate o-ethyltoluenefrom the lower boiling C aromatic hydrocarbons consisting of a mixtureof mand p-ethyltoluene, mesitylene and o-ethyltoluene. It has been foundthat if a mixture of the C aromatic hydrocarbons is contacted with apolyalkyl ester of an aromatic polycarboxylic acid having adjacentcarboxylic groups under vapor-liquid contacting conditions to form anoverhead Table II -OOOR -C O O R dlalkyl-o-phthalate C O O R COORdla1ky1-3-nitro-o-phthalate -C 0 O R OzN COORdia1ky1-4-nltro-o-phthalate C O O R COOR trialkyl hemlmellitate COORROOC COOR trialkyl trlmellitate ROOC COOR ROOC COOR tetraalkylpyromellitate where R is a C to C alkyl group and preferably a C te. Calkyl group.

In separating o-ethyltolucne in the extractive distillation process, thedesirable polyalkyl esters of aromatic acids are di'methyl-o-phthalate,dipropyl-3-nitrophthalate, tripropyl hemimellitate, timethyltrimellitate, and tetran-p-ropylpyromellitate and the like. It wasfurther found in separating o-et-hyltoluene in the extractivedistillation process that those compounds having three .adjacentsubstitutions on the aromatic ring were extractive solvents of adifferent order of magnitude than those having two adjacentsubstitutions. Thus, the dialkyl-3- nitro-o-phthalates and trialkylhemimellitates are superior to dialkyl-4-nitro-o-phthalates andtriallryl trimellitates, respectively. The solvent-to hydrocarbon ratiosused in the separation of o-ethyltoluene from the lower boiling Caromatic hydrocarbons may suitably range from 1:3 to 10:1 undervapor-liquid extractive distillation con ditions.

The present invention is considered to be the finding of suitablesolvents to separate the isomers of C aromatic hydrocarbons. Theextractive distillation techniques used in the separation of the Cisomers are those known to persons skilled in the art of carrying outextractive distillation operations. Thus, as is known in carrying outextractive distillations, the solvent used in an extractive distillationprocess must have a higher boiling point than the highest boilingproduct which is to be obtained in a separate fraction. In addition,there must be complete miscibility between the solvent and the materialsbeing separated at the temperatures existing in the extractivedistillation tower. It has been found, according to the presentinvention, that in the extractive distillation of the C isomers thepolyalkyl esters of the polycarboxylic acids alter the vapor-pressurecharacteristics of the C aromatic hydrocarbons so that the. relativevolatilities are altered. Thus, the relative volatilities are altered tothe extent that in the presence of the polyalkyl esters of thepolycarboxylic acids having adjacent carboxylic groups the isomers maybe separated into enriched fractions by using the techniques used inextractive distillation which are similar to that of a regulardistillation. The operating conditions in a regular distillation aresuch that the temperature is dependent upon variables governed mainly bythe pressure and the boiling point of the materials being separated.However, in an extractive distillation, such as in the extractivedistillation of C aromatics, the key to the extractive distillation isthe specific solvent such as the particular polyalkyl ester of thepolycarboxylic acid used since it is the choice of solvent which altersthe relative volatilities of the close boiling materials which enablesthe separation in the first place. The ratio of the particular solventto hydrocarbon employed, and in a minor extent the engineering factorsto be used in a specific separation, has an eifect on the conditions;but these elTec-ts are known to those skilled in the art. Thetemperature per se, of course, is not critical to the separation;however, it is obvious that a temperature above the boiling point of theclose boiling materials must be used to obtain an overhead fraction andthe vapor-liquid conditions. The pressure employed is normallyatmospheric, but a superatmospheric or subatmospheric pressure may beused in a particular operation since pressure, as such, is not criticalto the separation but merely changes operating conditions.

It was further found in accordance with another aspect of the inventionthat mand p-ethyltoluene could be separated from mesitylene byextractive distillation techniques using a C to C dialkyl ester ofo-phthalic acid. It is quite unusual that the same material will be anextractive solvent for two separations of an isomer mixture. Thus, thefinding that the C to C dialkyl esters of ophthalic acid can separatemand p-ethyltoluene from mesitylene as well as separate mesitylene andoethyltoluene is quite unexpected. For example, if a tetraalkylpyromellitate is used, no separation of mand pethyltoluene frommesitylene can be obtained. Since pethyltoluene and mesitylene haveboiling points which are within almost 5 F. of each other, a commercialregular distillation between these fractions would be extremelydiflicult. However, according to this aspect of the present invention,the mand p-ethyltoluene fraction may be separated from mesitylene byextractive distillation techniques very easily and inexpensively. Thepreferred solvent is dimethyl-o phthalate, and the solventto-hydrocarbonratios employed may suitably range from 1:3 to :1 and preferably 2:1under the vapor-liquid extractive distillation conditions.

I In finding that the C to C dialkyl esters of o-phthalic acid wouldenable the separation of mand p-ethyltoluene from mesitylene, as well asenabling the separation of mesitylene and o-ethyltoluene into separatefractions by extractive distillation techniques, another aspect of thepresent invention is a simplified process whereby these fractions can beseparated one from the other by using the same solvent in extractivedistillation zones while utilizing a single stripper for separating thesolvent from the hydrocarbon, thereby eliminating one solvent strippingtower. According to the simplified process, a mixture of the C isomersconsisting of mand p-ethyltoluene, mesitylene and o-ethyltoluene may beseparated in high purities by using two extractive distillations inseries without an intermediate solvent stripper using the same solventfollowed by a simple distillation to recover the solvent and to separatethe heavier C aromatic hydrocarbon as a high purity fraction if this isdesired. It isomers heavier than o-ethyltoluene are present, they may beseparated by a second simple distillation. The solvents used in thissimplified process are the C to C dialkyl esters of o-phthalic acid, thepreferred solvent being d-im-ethyl-o-phthalate.

The present invention will be further illustrated by reference to thedrawings in which:

FIG. 1 is a schematic flow diagram illustrating a simple extractivedistillation unit; and

FIG. 2 is a flow diagram of the simplified process for separating Cisomers of aromatic hydrocarbons.

Referring now to the drawings and particularly to FIG. 1, the drawingshave reference to a conventional extractive distillation apparatus whichmay be used according to the present invention to make a separation ofo-ethyltoluene from the lower boiling C aromatic isomers. The feedmixture consisting of about 25 mol percent p-ethyltoluene, about 50 molpercent rn-ethyltoluene, about 12 mol percent mesitylene, and about 13mol percent o-ethyltoluene is introduced into extractive zone 11 by feedline 12. The distillation zone 11 is a distillation tower provided withinternal vapor-liquid contacting means such as bell cap trays or otherequivalent means. The extractive distillation zone is also provided withmeans (not shown) for introducing reflux back into the tower andauxiliary condensing and cooling means. Further, the extractivedistillation zone 11 is provided with means illustrated by steam coil 13to control the conditions within the tower. The desired solvent, whichmay be tetra-n-propylpyromellitate, is introduced by line 14 incountercurrent flow to the distillation zone. An overhead fraction whichis enriched in the lower boiling C isomers, namely mand p-ethyltolueneand mesitylene, is withdrawn by line 15. The bottoms fraction isrecovered by line 16 which is enriched in o-ethyltoluene. The bottomsfraction is discharged by line 16 into a distillation zone 17 which maybe a simple distillation column containing internal vapor-liquidcontacting means such as bell cap trays and the like. The distillationzone 17 is equipped with the usual means for inducing reflux andprovided with auxiliary condensing and cooling means and provided withinternal heating means illustrated by steam coil 18. In the distillationzone 17 -a simple distillation of the o-ethyltoluene from the solventtakes place, and the o-ethyltoluene fraction is recovered by line 19while the solvent, tetra-n-propylpyromellitate, is recovered by line 20.The solvent is circulatedthrough line 20 into solvent line 14 forreintroduction into the extractive distillation zone 11. Makeup solventmay be introduced into line 14 by opening valve 21. The extractivedistillation zone 11 may be maintained under a wide range of operatingconditions depending on the specific polyalkyl ester of the presentinvention employed and on the degree of purity desired for theo-ethyltoluene fraction. Byway of illustration, however, temperaturesat'the top of the extractive distillation zone 11 may be within therange of 325 to 330 F. at atmospheric pressure with asolvent-tohydrocarbon ratio of 2:1 to obtain a fraction substantiallydepleted of o-ethyltoluene overhead.

As is known to those skilled in extractive"distillations, theextractivedistillation process is one wherein the fractions are enrichedrather than obtaining ideal separations. Accordingly, while it is notshown in FIG. 1, it will be understood that facilities will be providedfor removing any of the solvent carried over in the overhead fractionfrom the extractive distillation zone 11. Such facilities may include astripping zone such as described in the description of distillation zone17.

It was further found in accordance with the present invention that the Cto C dialkyl esters of o-phthalic acid were unique in separating mandp-ethyltoluene from mesitylene. Accordingly, a simplified process may beaccomplished by utilizing the scheme illustrated in FIG. 2 whichillustrates apparatus for separating mand p-ethyltoluene, rnesityleneand o-ethyltoluene as separate fractions. In accordance with this aspectof the present invention, the preferred solvent is dimethyl-o-phthalatealthough other dialkyl esters having 1 to 4 carbon atoms in the alkylgroup may be employed. The C feed may include all the C aromaticisomers, namely m-ethyltoluene, p-ethyltoluene, rnesitylene,o-ethyltoluene and pseudocumene, which is introduced into extractivedistillation zone 30 by feed line 31. The extractive distillation zone30 is suitably a distillation tower provided with the usual internalvapor-liquid contacting means and other auxiliary equipment. Thesolvent, dimethyl-o-phthalate, is introduced by line 32 near the upperportion of the extractive distillation zone 30. The conditions withinthe extractive distillation zone are controlled by means illustrated bysteam coil 33, the conditions within the extractive distillation zone 30being controlled in a manner known to those skilled in the art. Forexample, the conditions would be such that the temperatures at the topof this zone 30 would be within the range of about 320 to 325 F. atatmospheric pressure. An overhead fraction enriched in mandp-ethyltoluene is withdrawn from extractive distillation zone 30 by line34. The bottoms fraction is withdrawn from zone 30 by line 35.

The bottoms fraction which would contain the C isomers heavier than mandp-ethyltoluene and the solvent is introduced into a second extractivedistillation zone 36. Additional solvent is introduced near the upperportion of the second extractive distillation zone 36 by line 37.Extractive distillation zone 36 is similar to that of zone 30 and isprovided by the usual internal vaporliquid contacting means as well asmeans for inducing reflux and auxiliary condensing and cooling means andthe like. A suitable means for maintaining the conditions withinextractive distillation zone 36 is illustrated by steam coil 38. Theoverhead fraction eriched in mesitylene is withdrawn from zone 36 byline 39. The bottoms fraction is removed by line 40.

The bottoms fraction from zone 36 is enriched in oethyltoluene and theheavier C isomers along with the solvent, dimethyl-o-phthalate. Thisbottoms fraction is introduced into a stripper column 41 wherein anordinary distillation is carried out to separate the solvent from theremaining C aromatic isomers. The stripper column 41 may be a towerprovided with similar construction as the extractive distillation zones30 and 36. Accordingly, besides the other apparatus modifications, theconditions within the stripping tower 41 are maintained by means such asillustrated by steam coil 42. In the simple distillation carried out instripping tower 41, the aromatic hydrocarbons and other materials havinga boiling point lower than the solvent are removed overhead by line 43while the solvent is recovered by line 44. The solvent may be recycledby line 45 and reintroduced by lines 32 and 37 into extractivedistillation zones 30 and 36, respectively. The solvent may be purgedfrom the system 6 through line 46' by operating valve 47, and makeupsolvent introduced by line 48 which is controlled by valve49.

The overhead fraction'from the stripper column 41 may be introduced intoa second distillation tower 50 to obtain the o-ethyltoluene isomer inhigh purity. Distillation tower 50" may be provided similarly asstripper tower 41, and the conditions therein controlled by meansillustrated by steam coil 51. The overhead fraction, namelyo-ethyltoluene, is recovered by line 52 while the bottoms fraction isrecovered by line 53 which may contain pseudocumene and other heaviercomponents.

To illustrate the present invention, a binary mixture of1,3,5-trimethylbenzene (.mesitylene) and 1-methyl-2-ethylbenzene(o-ethyltoluene) is extractively distilled in the presence of a solventhaving the nature described herein with separation of the mixture intoits component parts. While these particular aromatic hydrocarbons boilat 328.5 and 329.3 F. in the absence of a solvent at atmosphericpressure, a good separation is possible in utiliz ing a polyalkyl esterof polycarboxylic acid having adjacent carboxylic groups according tothe present invention.

For example, in a plate column the relative volatilities of mand pethyltoluene and rnesitylene to o-ethyltoluene were determined. Withouta solvent, the relative volatility of mand p-ethyl-toluene too-ethyltoluene is 1.06 and for rnesitylene to o-ethyltoluene is 1.009;whereas with tetra-n-propylpyromellitate as a solvent, the relativevolatility of mand p-ethyltoluene to o-ethyltoluene is 1.18 and forrnesitylene to o-ethyltoluene is 1.12.

In a 50 plate Oldershaw column equipped with means for introducingpreheated solvent on the top plate, a mixture of rnesitylene andethyltoluene isomers is charged to the still pot and the hydrocarbonsset on total reflux. Hourly samples of overhead and bottoms were takenuntil the analyses of the samples from hour to hour remained constant.The relative volatilities of the compounds were then determined, and thedata indicate that when the solvent is a C to C dialkyl-o-phthalate, notonly separation of the rnesitylene and the o-ethyltoluene is obtainable,but also separation of mand p-ethyltoluene and rnesitylene isobtainable. For example, the relative volatility of p-ethyltoluene tornesitylene without the solvent is 1.058 and is increased to 1.1215 whendimethyl-ophthalate is present as the solvent in about 50 mol percentconcentration.

The present invention is quite important and useful and allows theobtaining of numerous advantages in that separation of the particular Caromatic hydrocarbons in purified condition is important. Theseparticular C aromatic hydrocarbons heretofore have not been separatedpractically, whereas now it is possible to obtain a good separationunder vapor-liquid separating conditions. The particular compounds inpurified condition are important and useful as chemicals and in themanufacture of other chemicals.

The nature and objects of the present invention having been completelydescribed and illustrated, what we wish to claim as new and useful andsecure by Letters Patent 1. A method for separating C aromatichydrocarbons which comprises extractively distilling a mixture ofhydrocarbons containing mand p-ethyltoluene, rnesitylene ando-ethyltoluene by contacting with a C to C dialkyl ester of o-phthalicacid in a first extractive distillation zone whereby an overheadfraction enriched in mand p-ethyltoluene and a bottoms fraction enrichedin mesitylene and o-ethyltoluene are formed, contacting said bottomsfraction with additional amounts of said ester in a second extractivedistillation zone under extractive distillation conditions whereby asecond overhead fraction enriched in rnesitylene and a bottoms fractionenriched in o-ethyltoluene are formed, and recovering saido-ethyltoluene from said second bottoms fraction by simple distillation.

2. A method according to claim 1 wherein said ester isdimethyl-o-phthalate.

3. A method for separating o-ethyltoluene from a mixture containing thelower boiling C aromatic hydrocarbons which comprises contacting saidmixture in an extraction zone under extractive distillation conditionswith a polyalkyl ester of an aromatic polycarboxylic acid havingadjacent carboxylic groups and having 1 to 8 carbon atoms in the alkylgroups to form an overhead fraction and a bottoms fraction, andrecovering o-ethyltoluene from said bottoms fraction.

4. A method in accordance with claim 3 wherein said ester istetra-n-propylpyromellitate.

5. A method for separating mand p-ethyltoluene from a mixture containingthe higher boiling C aromatic hydrocarbons which comprises contactingsaid mixture under extractive distillation conditions with a C to Cdialkyl ester of o-phthalic acid whereby an overhead fraction enrichedin mand p-ethyltoluene is formed.

6. A method in accordance with claim 5 wherein said ester isdimethyl-o-phthalate.

References Cited by the Examiner UNITED STATES PATENTS 2,721,170 10/1955Johnson 202-395 2,763,604 9/1956 Dorsey et a1 20239.S

NORMAN YUDKOFF, Primary Examiner.

1. A METHOD FOR SEPARATING C9 AROMATIC HYDROCARBONS WHICH COMPRISESEXTRACTIVELY DISTILLING A MIXTURE OF HYDROCARBONS CONTAINING M- ANDP-ETHYLTOLUENE, MESITYLENE AND O-ETHYLTOLUENE BY CONTACTING WITH A C1 TOC4 DIALKYL ESTER OF O-PHTHALIC ACID IN A FIRST EXTRACTIVE DISTILLATIONZONE WHEREBY AN OVERHEAD FRACTION ENRICHED IN M- AND P-ETHYLTOLUENE ANDA BOTTOMS FRACTION ENRICHED IN MESITYLENE AND O-ETHYLTOLUENE ARE FORMED,CONTACTING SAID BOTTOMS FRACTION WITH ADDITIONAL AMOUNTS OF SAID ESTERIN A SECOND EXTRACTIVE DISTILLATION ZONE UNDER EXTRACTIVE DISTILLATIONCONDITIONS WHEREBY A SECOND OVERHEAD FRACTION ENRICHED IN MESITYLENE ANDA BOTTOMS FRACTION ENRICHED IN O-ETHYLTOLUENE ARE FORMED, AND RECOVERINGSAID O-ETHYLTOLUENE FROM SAID SECOND BOTTOMS FRACTION BY SIMPLEDISTILLATION.